1. Field of the Invention
The present invention relates to a method for manufacturing a nitride semiconductor wafer or a nitride semiconductor device, the nitride semiconductor wafer or the nitride semiconductor device obtained thereby and a laser irradiation apparatus used therefor. More particularly, the present invention relates to a method for manufacturing a nitride semiconductor wafer or a nitride semiconductor device that is provided with the step of removing a disparate substrate made of a material different from nitride semiconductor.
2. Description of the Related Art
When manufacturing a nitride semiconductor wafer or a nitride semiconductor device, it may become necessary to separate nitride semiconductor layer from a disparate substrate that is made of such a material as sapphire or SiC. For example, a nitride semiconductor wafer made of a free-standing nitride semiconductor is generally manufactured by growing a thick nitride semiconductor layer over the disparate substrate such as sapphire, and then removing the disparate substrate. When an UV LED is manufactured, a device layer is formed intervening a GaN layer that would make an underlying layer for crystal growth over a disparate substrate such as sapphire, a support substrate such as CuW is fastened onto the top surface of the device layer, the disparate substrate is removed from the GaN underlying layer, and the GaN underlying layer is removed by polishing thereby to form the device, in a certain case.
As a method for removing the disparate substrate from the nitride semiconductor layer, following methods are known:    (1) To remove the disparate substrate by irradiating the interface between the disparate substrate and the nitride semiconductor with laser beam (Japanese Unexamined Patent Publication (Kokai) No. 2004-91278, Japanese Unexamined Patent Publication (Kokai) No. 2000-101139)    (2) To remove the disparate substrate by polishing.    (3) To form a lift-off layer between the disparate substrate and the nitride semiconductor, then remove the disparate substrate by applying a stress to the lift-off layer (Japanese Unexamined Patent Publication (Kokai) No. 2000-101139)    (4) To form a layer of such a material that can be removed by etching between the disparate substrate and the nitride semiconductor, then remove the disparate substrate by etching away the layer of this material (Japanese Unexamined Patent Publication (Kokai) No. 7-202265)
Among these methods, the method of lifting off the disparate substrate by means of laser irradiation is most widely employed since it does not require the use of a special material and allows easy application in a short period of time. In this method, laser beam of a wavelength that has high absorption rate with the nitride semiconductor is irradiated from the disparate substrate side, so as to decompose the nitride semiconductor located near the interface into nitrogen and metal.
When the disparate substrate is removed from the nitride semiconductor layer by irradiating with laser beam, however, there has been such a case as the nitride semiconductor is not fully decomposed in a portion of the wafer, or the nitride semiconductor layer turns into a state of being fused, and a part of the nitride semiconductor layer is broken and lifted off along with the disparate substrate. Since intensity of the laser beam is distributed in a certain pattern, the wafer is not irradiated with uniform intensity of the laser beam. As a result, there can occur such a case as the nitride semiconductor is not fully decomposed in a region of the wafer and a metal produced by the decomposition bonds the nitride semiconductor layer and the disparate substrate. In this case, a fused portion of the nitride semiconductor layer and the disparate substrate remains on the wafer surface without being removed. Under this condition, nitrogen gas generated through the decomposition of the nitride semiconductor layer presses the nitride semiconductor layer in the fused portion, thus making it likely that the nitride semiconductor layer is broken. Existence of the fused portion also makes it necessary to apply a mechanical stress to completely remove the disparate substrate, which may also break the nitride semiconductor layer, thus leaving fragments of the broken nitride semiconductor layer over the disparate substrate. Moreover, removal of the disparate substrate by the irradiation of laser beam results in unsatisfactory surface condition of the nitride semiconductor layer, which requires mechanical polishing to make the surface smooth.